House conservatives will gather in Baltimore later this week to plot their course and hear from speakers ranging from Newt Gingrich to Mitt Romney. But one likely 2008 candidate, Sen. John McCain, has decided to skip the gathering.

McCain was invited to attend the annual House conservative retreat, sponsored by the Heritage Foundation, but he didn’t even bother to respond to the invitation. Roll Call reported on McCain’s decision blow off House conservatives.

McCain campaign spokesman Danny Diaz did not address why his boss wouldn’t attend, or why he didn’t respond to the invite, but he said the Senator enjoys many friendships on Capitol Hill and has daily opportunities to reach out to conservative lawmakers.

McCain’s failure to appear at the retreat is yet another sign he is losing ground to his competition to win the support of House members. Romney has been on roll with House Republicans, announcing several supporters in recent weeks. Gingrich, meanwhile, has won praise for his inspirational speeches invoking conservative principles.

Rudy Giuliani was also invited to speak at the retreat, but he cited a scheduling conflict. He will be on the West Coast later this week.

One has to wonder why McCain is avoiding House conservatives, especially at a meeting he could use to improve his image. Instead, the Arizona senator once again befuddles the very people he needs most to win the GOP nomination.

Disclosure: Later this week I will start my new job at the Heritage Foundation, which is sponsoring the retreat.

UPDATE — 2:48 p.m.: Pat Hynes, who works for McCain, tells me that Phil Gramm will speak on McCain’s behalf at the conservative retreat, but the retreat‚Äôs sponsor says that Gramm was invited before McCain and Gramm will talk about policy, not the ‚Äô08 campaign.

According to the retreat’s schedule, Gramm will address the group of conservatives on Friday afternoon, offering his reflections on “advancing conservatism in the minority.” He will be introduced by his long-time friend and former aide, Republican Study Committee Chairman Jeb Hensarling (R.-Tex.).

Gramm doesn’t have an official role with McCain yet, Hynes says, but “he has already endorsed [McCain] and will be an integral part of the campaign.”

According to the Heritage Foundation, Gramm’s invitation had nothing to do with McCain’s invitation. Gramm has been a speaker at the retreat in the past and was among the first speakers invited to attend.

Brad Dayspring, RSC communications director, had this to say about the McCain-Gramm situation:

RSC Chairman Hensarling has a long relationship with Senator Gramm and is proud to consider him a mentor. Jeb has served in various capacities for the Senator, including as his Executive Director for the NRSC. He is thrilled that Senator Gramm — who will be representing himself — will be able to speak and offer guidance about the Conservative movement and other important issues.